You’ll have to add a submit event handler to the <form> element. In the event handler, you’ll then get the values from the textboxes, then do whatever computation you want.

This form also doesn’t need the action and the method attributes, since you’re not actually submitting data to some backend system.

document.getElementById('form').addEventListener('submit', function(e) {
e.preventDefault(); // This prevents the form from submitting data
const p1Age = document.getElementById('p1age').value;
const p1Height = document.getElementById('p1height').value;
// Do the same for the other ages and heights
// Then do computation on the numbers, then output
});